1. Field of the Invention
The present invention relates to a process for the production of chlorinated polyvinyl chloride resin. More particularly, it relates to a process for producing chlorinated polyvinyl chlorides which process prevents the formation of floating resin in the step of chlorination and yields a chlorinated polyvinyl chloride resin excellent in free flow property and in further, which is superior in initial coloring as well as heat stability at a time of molding with application of heat.
2. Prior Art
A chlorinated polyvinyl chloride resin (hereinafter referred to as xe2x80x9cCPVCxe2x80x9d) can be obtained by chlorination of polyvinyl chloride resin (hereinafter referred to as xe2x80x9cPVCxe2x80x9d). Processes for chlorinating PVC suspended in an aqueous medium are known in the prior art and described, for example, in Japanese Laid Open Patent specifications Kokai Sho 58-103507 (1983) and Sho 64-6002 (1989).
When CPVC is chlorinated in aqueous suspension, the resins tend to float up, and because of this tendency, in the step of discharging the resins from the reaction vessel after completion of the reaction, the resins may partly remain in the vessel undischarged, or in the step of transportation of resin slurries, these may cause pump cavitation troubles. For these and other reasons, productivity is low.
On the other hand, CPVC is characterized by its higher heat resistance as compared with PVC, and therefore is used in manufacturing pipes, joints, industrial plates, sheets or the like where heat resistance is required.
However, CPVC is inferior in heat stability to PVC, and therefore, it easily undergoes scorching during molding processes, and hence is not suited for long time processing for molding. Discoloration troubles thus may be encountered in transparent moldings production, for instance.
Further, CPVC easily becomes electrified and therefore its free flow property is not good. For this reason, these resins tend to cause blocking of the hopper of an extruder, disturbing the intended constant extrusion, among other problems.
For these reasons, as a measure for resin floating, a technology was proposed in Japanese Laid Open Patent specification Kokai Hei 9-95513 (1997). The proposed method comprises chlorinating a PVC obtained by suspension polymerization in the presence of a specific amount of a specific polyvinyl alcohol as a suspending agent. This method indeed improves productivity since the floating resin problem can be avoided.
However, this proposed method is not yet free from the problems of heat stability in molding processes and/or discoloration of moldings, for transparent applications. In addition, the free flow property problem is still to be solved and, therefore, a constant extrusion operation is often difficult to achieve because of blocking of the hopper of the extruder, among other problems.
It is an object of the present invention to provide a novel process for the production of chlorinated polyvinyl chloride resin (CPVC), which prevents resin floating in the chlorination process and gives CPVC having excellent free flow property and further showing good resistance to initial discoloration as well as good heat stability at the time of heating and molding processing.
As a result of an intensive study made by the inventors of the present invention to solve the above-mentioned problems, the inventors found that in chlorinating PVC in aqueous suspension, if the PVC has a specific average particle diameter as well as specific porosity, the resin floating problem at the time of chlorination can be prevented, and the CPVC obtained by this process has excellent free flow property and that, in addition, the CPVC shows good resistance to initial discoloration and good heat stability in the heating and molding process, and they have now completed this invention.
More precisely, the process for the production of CPVC according to the present invention is characterized in that a PVC having an average particle diameter not less than 150 xcexcm and porosity not less than 0.15 cc/g at 31-1,011 psi is suspended in an aqueous medium in a reaction vessel and the above-mentioned chlorination reaction of PVC is carried out in said aqueous suspension.
By this invention, a process for the production of CPVC can be provided, which prevents resin floating in the chlorination process and gives CPVC having excellent free flow property and further showing good resistance to initial discoloration as well as good heat stability in the heating and molding process.